1. Field of the Invention
The treatment of cellulosic fiber with ozone.
2. Prior Art
Historically, the treatment of wood chips to form a white fiber has been divided into two processes, pulping and bleaching. Recently the distinction between these processes has become less distinct and the words have become more terms of art than a description of a chemical process. To provide a background for this invention, the two processes will be defined and distinguished. The present definitions are based upon the definitions provided in a number of pulping and bleaching textbooks and monographs.
Pulping is the changing of wood chips or other wood particulate matter to fibrous form. Chemical pulping requires cooking of the chips in solution with a chemical and includes partial removal of the coloring matter such as lignin associated with the wood.
Bleaching is the treatment of cellulosic fibers to remove or alter the coloring matter associated with the fibers to allow the fiber to reflect white light more truly.
Attempts to bleach cellulosic fiber with ozone, actually air or oxygen containing some ozone, have occurred since late 1800s. Many conditions have been tried and from these there has evolved a theory, substantiated by experiments, as to the best conditions for the ozonization of cellulose.
The principal work has been done by Doree with Cunningham in 1912 and 1913 and with Healey in 1938; Brabender et al in 1949; Osawa and Schuerch et al of Syracuse University in the 1960s; Liebergott et al of the Pulp and Paper Research Institute of Canada in the 1960s and 1970s; and Soteland et al of the Norwegian Pulp Research Institute in the 1960s and early 1970s.
The references describing this work are: Cunningham and Doree, "The Action of Ozone on Cellulose," Part I, Cotton and Part II, Jute, The Journal of the Chemical Society, Vol. 101 (1912), pp. 497-512, and Part III, Beechwood, The Journal of the Chemical Society, Vol. 103 (1913), pp. 677-686; Doree and Healey, "The Action of Ozone on Cellulose and Modified Cellulose," The Journal of the Textile Institute, March 1938, pp. T27-T42; Brabender et al, U.S. Pat. No. 2,466,633, 1949, "Method of Bleaching Cellulosic Pulp"; Pancirolli, "Sulphate Pulp Bleaching Tests With Ozone," Indi Carta (Milan), March 1953, pp. 35-38; Osawa and Schuerch, "The Action of Gaseous Reagents on Cellulosic Materials, Part I," TAPPI (1963), Vol. 46, No. 2, pp. 79-84; Schuerch "Ozonization of Cellulose and Wood," Journal of Polymer Science, Part C, No. 2, 1963, pp. 79-95; Soteland (I), "The Effect of Ozone on Some Properties of Groundwood of Four Species, Part I," Norsk Skogindustri, March 1971, pp. 61-66; Secrist and Singh, "Kraft Pulp Bleaching II: Studies on the Ozonation of Chemical Pulps," TAPPI, Vol. 54, No. 4, April 1971, pp. 581-584; Liebergott "Paprizone Process for Brightening and Strengthening Groundwood," Paper Trade Journal, Aug. 2, 1971, pp. 28-29; Soteland (II) "Bleaching of Chemical Pulps with Oxygen and Ozone," Pulp andd Paper Magazine of Canada, Vol. 75, No. 4, April 1974, pp. 91-96; and Procter, "Ozone gas treatments of high Kappa kraft pulps," Pulp and Paper Magazine of Canada, Vol. 75, No. 8, June 1974, pp. 58-62.
From these publications a consensus can be seen. High consistencies are required to treat cellulosic fiber, either cotton or wood pulp, with ozone. The exact percentages may differ slightly, but the message that high consistencies are required is emphatic. There is some slight confusion because the figures are either in terms of moisture content--amount of water on the fiber--or consistency--amount of fiber in the water. The Doree articles indicate that cotton requires a 50% moisture content for good ozonization. Procter indicates this is the same as 67% consistency. Brabender states that for wood fibers 25 to 55% consistency is required. This was later amended by Osawa and Schuerch to 30 to 45% consistency--230 to 120% moisture content. Osawa and Schuerch then used 100% moisture content for a number of experiments. Liebergott, treating mechanical pulp in which the chemical reaction with ozone appears to be different from the reaction with chemical pulp, used consistencies of 15 to 60%. Secrist and Singh tried the consistencies of 40 to 80%, preferring 60%. Procter notes that 30 to 40% consistency with wood pulp fibers is best.
Only a few have attempted to ozonate at low consistencies. The results are not considered successful, and the experimenters returned to higher consistencies as a matter of standard practice.
Three articles discuss work at low consistency.
Soteland treated pulp in a 90% by volume acetone solution at a 0.5% consistency. He indicates that pulp at low consistency can only be treated in an organic solution.
Pancirolli attempted ozonization of sulphate pulp at 2% consistency. It required three treatments of five hours each for a total of 15 hours.
Schuerch amplifies a statement made in the Osawa and Schuerch article about low-consistency work, and states that ozonization was carried out at consistencies of 0.1% and 1%. FIG. 4 of Schuerch indicates that at 0.1% consistency, the brightness, initially 30, increased to between 50 and 60 in ten minutes, between 75 and 80 in one-half hour, and around 81 or 82 in one hour. However, at 1% consistency the brightness increased to 60 in one hour and required three hours to finally reach 80, even with "vigorous stirring." From this he concluded that one had to use organic substances or higher consistencies to get good reaction with ozone.
These comments were echoed by Liebergott, Soteland and Procter in their work and articles. It was considered by all to be impossible to obtain quick, good reactions with ozone at low consistencies.
A recent patent, Oldshue U.S. Pat. No. 2,966,542 issued June 29, 1976, describes a multi-stage chlorination system but indicates that the system can be used for ozone. This patent stages that reaction time is independent for power level after a certain threshold power level has been reached.
Oldshue specifies, in line 9 of column 7, a consistency of 3.5%. His power levels, in the table at the bottom of column 6, are 20 to 60 horsepower per 100 gallons, equivalent to 1.5 to 4.5 horsepower per cubic foot.
While none of the prior art describes an ozone treatment in a low-consistency water solution in conjunction with other treatments, a number of the articles describe high-consistency ozone treatments in conjunction with other pulp treatments. Four of these appear to be pertinent. These are Secrist and Singh, supra; Soteland (II), supra; Singh Canadian Patent No. 966,604, 1975, "Kraft Pulp Bleaching and Recovery Process"; and Rothenburg et al "Bleaching of Oxygen Pulps with Ozone," TAPPI, Vol. 58, No. 8, August 1975, pp. 182-185.
Secrist and Singh mention an O.sub.3 DED sequence--ozone, chlorine dioxide, sodium hydroxide extraction, chlorine dioxide.
Soteland (II) mentions a number of sequences. These include ozone-peroxide, ozone-hypochlorite, ozone-ozone, oxygen-ozone, oxygen-ozone-peroxide, oxygen-ozone-hypochlorite, oxygen-ozone-ozone-peroxide, and oxygen-ozone-ozone-hypochlorite. Soteland treats his pulp with sulfur dioxide and EDTA prior to the ozone treatment.
Singh mentions kraft-ozone-sodium hydroxide extraction-peroxide. The ozone may be in one, two, or three stages with an optional washing between the ozone stages.
Rothenburg describes oxygen-ozone, oxygen-ozone-sodium hydroxide extraction-ozone, oxygen-ozone-peroxide, oxygen-ozone-acetic acid, and kraft-ozone-sodium hydroxide extraction-ozone.
Again it should be emphasized that these ozone treatments were high-consistency treatments, and the use of high-consistency treatments created another problem, erratic results and poor strength properties.
The strength properties are mentioned in a number of patents and articles.
Pancirolli notes on page 8:
"Tests demonstrated that sulphate pulp can be bleached with ozone alone but with a notable reduction of the final pulp viscosity, in physical and mechanical properties as well as in the yield." PA1 "Tearing strength of the ozonated pulp was 10% lower than conventional fiber at both reported freeness levels. The same relationships was apparent when the pulps were compared at constant breaking length levels of 7,500 and 11,500. The interrelationship of fiber bonding with tearing energy may explain these observations." PA1 "It is evident that bleaching methods based on oxygen, ozone, and peroxide produce pulps with viscosity values far below what is common for conventional pulps. PA1 "Secrist and Singh have shown, however, that even if the viscosity is drastically reduced by an ozone treatment, the strength properties of the kraft pulp were not seriously affected. The tear factor of this eucalypt kraft pulp has been substantially reduced by this bleaching treatment. The drop in tear factor is too serious for this particular pulp for an acceptance of the oxygen-ozone bleaching process as presented here. However, it has to be stressed that this oxygen-ozone bleaching process is still in its stage of birth and improvement are to be expected."
This is also illustrated in a table in which the viscosity of ozone-bleached pulp is 15 and 21 centipoises compared to a viscosity of 50 centipoises for pulp bleached with chlorine and hypochlorite. In a comparison of the ozone-bleached pulp with the chlorine/hypochlorite-bleached pulp at the same brightness, the breaking length, the burst, and the fold of the ozone-treated pulp were less than those of the chlorine/hypochlorite-bleached pulp.
Katai and Schuerch, on page 2695 of their article "Mechanism of Ozone Attack on Alpha Methyl Glucoside and Cellulosic Materials" in the Journal of Polymer Science, Part A 1, Vol. 4, pp. 2683-2703 (1966), show that the viscosity decreases greatly as the brightness of the pulp increases when being treated with ozone.
Although the strength properties of groundwood pulps are usually increased by ozone treatment because of the modification of both the lignin and the surface of the fibers, allowing better bonding, chemical pulps do not appear to react in the same manner.
Secrist and Singh tested Canadian hardwoods. Although Table 1 and Table 2 appear to show no difference in tear between the control and ozone-treated samples, Table 4-6 appear to show that the kraft O.sub.3 DED sequences have a lower tear than the kraft CEDED sequences. On page 583 it is stated:
The article also indicates there is no relationship between viscosity drop and strength.
The Soteland (II) article states that ozone is more a delignifying agent than a bleaching agent. In the first paragraph on page 93 he notes:
He also worked with sodium bisulfite pulp from spruce and found the strength properties more satisfactory. The tear factor was reduced but the decrease was rather small, and therefore not prohibitive for the acceptance of these bleaching methods for sulfite pulps. He obtained the same viscosity of around 700 cubic centimeters per gram, using two-stage ozone, ozone plus peroxide, or oxygen plus ozone.
Procter in FIG. 1 shows that ozone treatment reduces tear. Tear is low at 30% consistency, but higher at 15consistency, the lowest consistency shown. Procter states that these sheet properties corresponding to carbohydrate reactions are most significantly altered when ozonizations are carried out at between 30 and 35% consistency where burst, strength, tensile and density are at a maximum and tear factor is at a minimum.
Kamaslimi et al, "Ozone bleaching of Kraft Pulp," 19th Japanese Symposium on the Lignin Chemistry, 1974, shows in FIG. 7 that tear, burst, and breaking length decrease as the ozone supply increases.
Rothenburg et al seems to indicate that results using high-consistency ozone bleaching are not consistent.